Nanomechanical standards based on the intrinsic mechanics of molecules and atoms

For more than a decade, instruments based on local probes have allowed us to “touch” objects at the nanoscale, making it possible for scientists and engineers to probe the electrical, chemical, and physical behaviors of matter at the level of individual atoms and molecules. In principle, physical interactions on this scale are characterized by fixed, unique values that need only be reliably measured in terms of accurately realized units of force and length to serve as standards. For example, the silicon lattice spacing is often used as a convenient ruler for estimating length in atomic scale images, since this lattice spacing has been independently measured using x-ray interferometry. Recently, the force-induced failure of DNA, often referred to as the overstretch condition, has been proposed as both a standard of force and length in single-molecule bio-physics experiments. Still other nanomechanics researchers have suggested that the rupture force of a single-atom chain is unique to a given metal, and that this intrinsic force can be used to calibrate atomic break junction experiments. In both these examples, a fundamental assumption is that the irreducible nature of nanoscale experimentation, in this case tensile testing, yields consistency befitting a standard. This paper offers context and a condensed overview of recently published results from the NIST Small Force Metrology Laboratory regarding new instruments and capabilities we have developed to examine this fundamental assumption. The reviewed papers describe new test platforms, techniques, and calibration procedures that allow us to bring accurate picoscale measurements of both length and force to bear on the problems of single-molecule and single-atom tensile testing.